The invention relates to a new high speed steel suited for tools the use of which requires a high toughness in combination with a hardness and strength suitable for the application in question. Typical applications are dies for the extrusion of aluminum profiles, qualified machine elements and pressure rolls, i.e. tools for embossing patterns or profiles in metals, etc. Another field of application is tools for cutting working, e.g. thread cutting taps and end-cutters with chip breakers, which require a high toughness in combination with a high hardness, particularly a high hot hardness.
One of the most important features of a steel which shall be used, e.g., for tools for the extrusion of aluminum profiles is that the steel is tempering resistant, which means that it shall be able to be exposed to a high temperature during a long period of time without losing the hardness which the steel has obtained after hardnening and tempering. On the other hand, this hardness need not be extremely high, suitably being in the range 50-55 HRC.
A high hardness and strength in combination with a high toughness are primary features if the steel instead shall be used for qualified machine elements. In this case, the hardness after tempering typically may be in the range 55-60 HRC.
Still higher demands upon hardness, 60-67 HRC, even in combination with a high toughness are raised on steels for tools intended for embossing patterns or profiles in metals, etc., and on steels for tools for cutting working, i.e. thread cutting taps and end cutters with chip breakers. Thread cutting taps should have a hardness in the range 60-65 HRC while end cutters should have a hardness in the range 62-67 HRC.
For applications of the above mentioned kind there are contemporarily usually used such tools steels as hot work steels, qualified constructionsteels and sometimes high speed steels. An example of a high speed steel for this type of application is the commercial high speed steel which is known under its trade name ASP.RTM. 23 (currently available from Erasteel Kloster Aktiebolag, a Swedish corporation), which is characterized by the following nominal composition in weight-%: 1.29 C, 0.4 Si, 0.3 Mn, 4.0 Cr, 5.0 Mo, 6.2 W, 3.1 V, balance iron and unavoidable impurities. Another high speed steel which is used, e.g., for cutting working, is ASP.RTM. 30 (currently available from Erasteel Kloster Aktiebolag, a Swedish corporation), which has the nominal composition 1.28 C, 4.2 Cr, 5.0 Mn, 6.4 W, 3.1 V, 8.5 Co, balance iron and unavoidable impurities. All percentages relate to weight-%.
The above-mentioned commercially available steels ASP.RTM., 23 and ASP.RTM., 30 have a considerable toughness in comparison with other high speed steels but do not completely satisfy those demands which are raised on materials, e.g., for the above mentioned applications, nor do any other commercial steels currently exist which fully satisfy all the said demands. The purpose of the invention is to provide a new high speed steel which more satisfactorily fulfills these demands. More particularly, the steel shall have the following features:
it shall have a high toughness in the hardened condition; PA1 a hardness of max 250 HB before hardening; PA1 a good hardenability, including precipitation hardenability to a hardness between 50 and 67 HRC suitable for the application in question, by choice of a hardening temperature between 925 and 1225.degree. C. and subsequent tempering; and PA1 a high toughness in the hardened and tempered condition by the fact that the steel contains a comparatively small total amount of carbides, max 5% by volume, that the carbides are small and evenly distributed, that the microstructure is fine grained corresponding to austenite grains having sizes corresponding to Intercept&gt;20 according to Snyder-Graff), and that it has a low content of retained austenite. PA1 firstly, the steel is alloyed with niobium and with a sufficient amount of carbon--as far as carbon is concerned, se supra--in order to provide a sufficient amount of niobium carbides, NbC, which are not dissolved to a substantial degree at the above mentioned, high temperature but will remain undissolved to work as grain growth inhibitors. PA1 secondly, measurements are taken in order that the primary niobium carbides be small and evenly distributed in the steel, which is a condition for their ability to work as grain growth inhibitors. This condition is satisfied by the powder-metallurgical manufacturing, which garantees that the niobium carbides be small and evenly distributed.
These and other conditions may be satisfied if the steel is given a balanced alloy composition according to the appending claims. In the following, the choice of the various alloy elements will be discussed. Herein, some theories will be mentioned concerning mechanisms which are considered to be the basis for the achieved effects. It shall, however, be noted that the claimed patent protection is not bound to any particularly theory.